Blank space appears on a docked window when it's expanded horizontally

Steps to Repro:
1. Create a new project
2. Select any docked window, "Inspector" for instance
3. Drag it horizontally few times

Actual Result:
A blank space appears on the right-hand side of the window

Expected Result:
The expanded window should completely be visible when expanded horizontally

Environment:
Windows and Mac

Occurring with:
2018.3.0a4, 2018.3.0a3, 2018.3.0a2

Working with:
2018.3.0a1, 2018.2.0b10